使用 G1GC 時的隨機高 Ref Proc

Question

由於我的應用程序需要低延遲（最大 10 毫秒），我使用 G1GC 與非常低的 Eden (10mb) 和低堆內存 (250mb)。 目標是經常進行大量次要 GC，但持續時間非常短。 我正在使用以下配置：

-Xms250m \
-Xmx250m \
-XX:+PrintGCDetails -Xloggc:/tmp/myapp-gc.log \
-XX:+PrintGCTimeStamps \
-XX:+PrintGCApplicationStoppedTime \
-XX:+PrintSafepointStatistics \
-XX:+NeverTenure \
-XX:-UseBiasedLocking \
-XX:-UsePerfData \
-XX:+UseG1GC \
-XX:-ResizePLAB \
-XX:-UseStringDeduplication \
-XX:+UnlockExperimentalVMOptions \
-XX:G1NewSizePercent=5 \
-XX:G1MaxNewSizePercent=5 \
-XX:MaxMetaspaceSize=200m \
-XX:ParallelGCThreads=4 \
-XX:ConcGCThreads=1 \
-XX:GCTimeRatio=2 \
-XX:InitiatingHeapOccupancyPercent=50 \
-XX:+DisableExplicitGC \
-XX:+ParallelRefProcEnabled \
-XX:+UseFastAccessorMethods \
-XX:+UseAltSigs \

我執行了 1 小時的測試，似乎達到了我的目標，但由於某些原因，我有一些 Minor GC 需要長達 30 毫秒（而平均值為 5 毫秒）。

下面是 1h 測試的統計數據：

No.     of GCs  Percentage
0 - 10  9550    99.29%
10 - 20 66      0.69%
20 - 30 2       0.02%
Avg Pause GC Time   4.66 ms

也是一個“正常的”次要 GC 和一個 Ref Proc 太高的：

11601.845: [GC pause (G1 Evacuation Pause) (young), **0.0035909 secs**]
   [Parallel Time: 2.2 ms, GC Workers: 4]
      [GC Worker Start (ms): Min: 11601844.9, Avg: 11601845.0, Max: 11601845.4, Diff: 0.6]
      [Ext Root Scanning (ms): Min: 0.5, Avg: 1.3, Max: 2.0, Diff: 1.5, Sum: 5.2]
      [Update RS (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.2]
         [Processed Buffers: Min: 0, Avg: 5.0, Max: 14, Diff: 14, Sum: 20]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.4]
      [Termination (ms): Min: 0.0, Avg: 0.5, Max: 0.7, Diff: 0.7, Sum: 1.9]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 4]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 1.5, Avg: 1.9, Max: 2.1, Diff: 0.6, Sum: 7.7]
      [GC Worker End (ms): Min: 11601847.0, Avg: 11601847.0, Max: 11601847.0, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 1.3 ms]
      [Choose CSet: 0.0 ms]
      [**Ref Proc: 1.0 ms**]
      [Ref Enq: 0.1 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 11264.0K(11264.0K)->0.0B(11264.0K) Survivors: 1024.0K->1024.0K Heap: 88152.5K(250.0M)->76944.6K(250.0M)]
 [Times: user=0.01 sys=0.00, real=0.00 secs]
11601.849: Total time for which application threads were stopped: 0.0041718 seconds, Stopping threads took: 0.0000731 seconds 

問題一：

6435.887: [GC pause (G1 Evacuation Pause) (young), **0.0356522 secs**]
   [Parallel Time: 4.0 ms, GC Workers: 4]
      [GC Worker Start (ms): Min: 6435886.8, Avg: 6435888.6, Max: 6435890.4, Diff: 3.6]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 1.3, Max: 2.6, Diff: 2.6, Sum: 5.2]
      [Update RS (ms): Min: 0.0, Avg: 0.2, Max: 0.5, Diff: 0.5, Sum: 0.8]
         [Processed Buffers: Min: 0, Avg: 4.0, Max: 14, Diff: 14, Sum: 16]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 0.8]
      [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 0.6]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 4]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 0.0, Avg: 1.9, Max: 3.8, Diff: 3.8, Sum: 7.5]
      [GC Worker End (ms): Min: 6435890.4, Avg: 6435890.5, Max: 6435890.6, Diff: 0.2]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.0 ms]
   [Other: 31.6 ms]
      [Choose CSet: 0.0 ms]
      [**Ref Proc: 31.3 ms**]
      [Ref Enq: 0.1 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 10240.0K(10240.0K)->0.0B(10240.0K) Survivors: 2048.0K->2048.0K Heap: 82113.0K(250.0M)->71825.0K(250.0M)]
 [Times: user=0.02 sys=0.00, real=0.03 secs]
6435.922: Total time for which application threads were stopped: 0.0362249 seconds, Stopping threads took: 0.0000478 seconds 

關於如何解決或克服此問題的任何想法/建議？

我正在使用 java-1.8.0-openjdk-1.8.0.222.b03-1.el7.x86_64 和 wildfly-10.1.0。 目前更新不是一個選項。

Answer 1

分析內存堆后，主要問題與終結器的弱引用類型有關。

所有這些都以 com.sun.jndi.ldap.LdapSearchEnumeration 為參照（因為在我們的應用程序中，我們使用 LDAP 搜索和 jndi-ldap）。

改為UnboundID LDAP SDK后，問題解決。

此外，如果有人感興趣，在 JDK8 上我們使用 Shenandoah GC 獲得了最佳性能。